In vitro and in vivo experimentation has shown that infection with murine sarcoma virus (Mo-MSV (MLV)) resulted in rapid decreases in amounts of cytoplasmic ribonuclease activity which correlated inversely with levels of viral specific cytoplasmic mRNA and with concentrations of reverse transcriptase and p30 associated with the microsomal fractions. Similar changes occurred when mice were infected with murine leukemia virus (Mo-MLV) at the later periods when leukemia began to be observed. Injections of Poly A:U suppressed virus expression in spleen lymphocytes.